No matter how often that myth is repeated - the adjustable rate on PS/2 is NOT an advantage, but merely a kludge to let the user balance between smooth mouse movement with high system load and jerky pointer motion at low system load. Note that fast CPUs don't help, since the access times toward this historically ancient piece of silicon that is the keyboard/mouse controller (we're talking 1977, 8-bit ISA here!) are astronomical.
On USB, which is a bus mastering PCI device, smooth pointer movement and low system load aren't contradictory goals, so there simply is no NEED to make anything adjustable.
USB. Twenty years of progress over PS/2.